Conventional silver halide emulsions are frequently prepared by means of one of two precipitation methods followed by a physical ripening period. In the single-jet precipitation method, silver nitrate solution is added to a solution containing a protective colloid and halide ions. Emulsions prepared by this method usually have large grains which vary considerably in their crystal habit and size distribution. However, this method is capable of producing large-grain sensitive emulsions. In the double-jet precipitation method, silver nitrate solution and soluble halide solution are added simultaneously to a solution containing a protective colloid. Emulsions prepared by this method under controlled conditions can have grains with a narrow size distribution, i.e., the emulsion is monodisperse and has a uniform crystal habit, e.g., predominantly regular grains which have no twin-plane stacking faults.
The use of ripening agents in the preparation of photographic silver halide emulsions by either of the foregoing methods is old in the art. Thus, silver halide grains have been precipitated and ripened in the presence of excess halide ion and/or ammonia to modify grain size, grain size distribution, and morphology. Such agents, however, restrict the pH or pAg range in which the silver halide grains can be formed. Furthermore, because ammonia is volatile, control of concentration of growth agent, and hence, control of silver halide grain formation, is difficult.
The use of thiourea and other compounds containing hetero atoms as physical ripening accelerators has been disclosed by Oh-Yama in Proceedings of the Royal Photographic Society, Centenary Conference, London, 1953, pp. 37-42, published 1955. Although such substances promoted increased grain size, they were reported to have no practical utility, because they also produced heavy fog in actual emulsions.
There remains, therefore, a need for agents which can be used to promote the growth of silver halide grains of increased and uniform size and uniform crystal habit, which agents can be used in the precipitation and/or ripening steps over a range of conditions of pH, pAg and temperature with ready control of their concentration and without undesirable effects on the photographic performance of the grains or of emulsions containing the grains.